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Synthesis Study of an Erosion Hot Spot, OceanBeach, California

Patrick L. Barnard{, Jeff E. Hansen{{, and Li H. Erikson{

{United States Geological SurveyPacific Coastal and Marine Science Center400 Natural Bridges DriveSanta Cruz, CA 95060, [email protected]

{University of California Santa CruzDepartment of Earth and Planetary Sciences1156 High StreetSanta Cruz, CA 95064, U.S.A.

ABSTRACT

Barnard, P.L.; Hansen, J.E., and Erikson, L.H., 2012. Synthesis study of an erosion hot spot, Ocean Beach, California(USA). Journal of Coastal Research, 28(4), 903–922. West Palm Beach (Florida), ISSN 0749-0208.

A synthesis of multiple coastal morphodynamic research efforts is presented to identify the processes responsible forpersistent erosion along a 1-km segment of 7-km-long Ocean Beach in San Francisco, California. The beach is situatedadjacent to a major tidal inlet and in the shadow of the ebb-tidal delta at the mouth of San Francisco Bay. Ocean Beach isexposed to a high-energy wave climate and significant alongshore variability in forcing introduced by varying nearshorebathymetry, tidal forcing, and beach morphology (e.g., beach variably backed by seawall, dunes, and bluffs). In addition,significant regional anthropogenic factors have influenced sediment supply and tidal current strength. A variety oftechniques were employed to investigate the erosion at Ocean Beach, including historical shoreline and bathymetric analysis,monthly beach topographic surveys, nearshore and regional bathymetric surveys, beach and nearshore grain size analysis,two surf-zone hydrodynamic experiments, four sets of nearshore wave and current experiments, and several numericalmodeling approaches. Here, we synthesize the results of 7 years of data collection to lay out the causes of persistent erosion,demonstrating the effectiveness of integrating an array of data sets covering a huge range of spatial scales. The key findingsare as follows: anthropogenic influences have reduced sediment supply from San Francisco Bay, leading to pervasivecontraction (i.e., both volume and area loss) of the ebb-tidal delta, which in turn reduced the regional grain size and modifiedwave focusing patterns along Ocean Beach, altering nearshore circulation and sediment transport patterns. In addition,scour associated with an exposed sewage outfall pipe causes a local depression in wave heights, significantly modifyingnearshore circulation patterns that have been shown through modeling to be key drivers of persistent erosion in that area.

ADDITIONAL INDEX WORDS: Erosion, modeling, beach, waves, erosion hot spot.

INTRODUCTION

Understanding the physical processes driving the persistence

of local coastal erosion is key to effective mitigation, particularly

in urban areas, where high-value infrastructure is often at risk.

In addition to understanding the physical processes occurring

directly where erosion is occurring, knowledge of the larger

spatial and temporal processes is essential. Examples of

important, larger-scale processes include natural and anthropo-

genic influences on sediment supply and pathways (e.g., Frihy

and Komar, 1993; Jabaloy-Sanchez et al., 2010; Slagel and

Griggs, 2008), bathymetric change (e.g., Elias and van der Spek,

2006), shoreline and cliff change (e.g., Hapke, Reid, and

Richmond, 2009; Jones et al., 2009), tidal currents (e.g., Davis

and Barnard, 2003), wave-focusing patterns (e.g., Shi et al.,

2011), and wave climate trends (e.g., Allan and Komar, 2006;

Ruggiero, Komar, and Allan, 2010). In coastal settings adjacent

to river deltas or ebb-tidal deltas, the interaction among sediment

supply, wave climate, and tidal currents controls the evolution of

the bathymetry (Wright and Coleman, 1972), which in turn

exerts a first-order control on the nearshore coastal processes

(e.g., Davis and Fox, 1981) and shoreline behavior (e.g., Barnard

and Davis, 1999; Davis and Barnard, 2000; El Banna and Frihy,

2009; Fan, Huang, and Zeng, 2006; Mateo and Siringan, 2007).

Erosion ‘‘hot spots’’ are herein defined as local sections of

beach—O(100 to 1000) m alongshore—that show persistent

multiannual to decadal anomalous erosion or higher erosion rates

compared to the adjacent beach (Bridges, 1995; Dean, Liotta, and

Simon, 1999; Stauble, 1994). Hot spots have been linked to (1)

alongshore-varying nearshore bathymetry, often associated with

tidal inlets (e.g., Bruno, Yavary, and Herrington, 1998; Hicks

et al., 1999), river mouths (e.g., Barnard and Warrick, 2010),

dredge borrow pits (e.g., Benedet and List, 2008), transverse bars

(e.g., Hapke et al., 2010; Konicki and Holman, 2000), or bedrock

irregularities (e.g., McNinch, 2004), or (2) shadowing effects

associated with coastal headlands and structures, such as

breakwaters, groins, and jetties, all of which directly interrupt

alongshore transport (Kraus and Galgano, 2001).

An erosion hot spot has persisted for decades in the southern

reach of Ocean Beach in San Francisco, California, fronting

critical wastewater infrastructure for the City of San Fran-

cisco. Beach and bluff loss from storms during winter 2009–10

led to $5 million in emergency remediation efforts in the hot-

spot area, and continued erosion could lead to significant

impacts along this shoreline. Hard structures that impede

littoral transport, such as breakwaters, groins, and jetties, are

DOI: 10.2112/JCOASTRES-D-11-00212.1 received 22 November 2011;accepted in revision 21 December 2011.’ Coastal Education & Research Foundation 2012

Journal of Coastal Research 28 4 903–922 West Palm Beach, Florida July 2012

nonexistent along Ocean Beach; thus, alongshore variations in

the nearshore bathymetry, as well as tidal and wave forcing

associated with natural geography, are expected to be the

major contributing factors to the hot-spot erosion. In this paper,

we synthesize the findings of a variety of local and regional

analyses to describe the primary contributors to persistent

erosion in the southern end of Ocean Beach. Our investigation

highlights the importance of a comprehensive regional ap-

proach to understanding the processes controlling the behavior

of a discrete section of coast. Furthermore, this work directly

supports key management questions regarding the mitigation

of coastal erosion and the protection of valuable infrastructure.

STUDY AREA

Ocean Beach is a 7-km-long, sandy (d50 5 0.3 mm) beach

located on the U.S. West Coast in the center of the San Francisco

Littoral Cell, which extends from Point Reyes in the north to

Point San Pedro in the south (Figure 1). Ocean Beach trends

north–south along the southern perimeter of the Golden Gate

inlet, the sole tidal inlet emptying San Francisco Bay, where tidal

currents exceed 2.5 m/s during peak tidal flows in the inlet throat

and reach 1.5 and 0.5 m/s at the extreme north and south ends of

the beach, respectively (Barnard et al., 2007a). A massive ebb-

tidal delta (.150 km2) is located seaward of the Golden Gate,

exerting dominant control on regional wave patterns (Eshleman

et al., 2007). Bathymetric features within the ebb-tidal delta that

directly influence coastal morphodynamics at Ocean Beach

include a 1.5-km-wide longitudinal bar along the southern lobe

of the delta that merges with the nearshore in the center of the

beach, a flood channel that cuts across the outer surf zone to the

north, and a 200-m-wide scour pit associated with an exposed

sewage outfall pipe offshore to the south (Figure 1). These

features produce irregular alongshore bathymetry and influence

physical forcing. The variable nearshore morphology and strong

tidal currents cause complex patterns of wave focusing and

refraction, alongshore wave height variability (Eshleman

et al., 2007), and wave–tidal current interaction.

The regional mean annual deep-water significant wave height

Hs is 2.5 m, with a maximum of 9.1 m at the Point Reyes wave

Figure 1. Overview of the study area.

904 Barnard, Hansen, and Erikson

Journal of Coastal Research, Vol. 28, No. 4, 2012

buoy (Table 1 and Figure 2; Scripps Institution of Oceanography,

2011). The dissipation of deep-water waves by the shelf reduces

Hs by about 25%, although nearshore in 11.5-m water depth in

the middle of Ocean Beach, the maximum wave height Hmax has

been measured as more than 10 m due to local focusing (Hansen

and Barnard, 2010). Swell typically approaches the region from

the NW and W, but S swell can occur during the summer months.

Ocean Beach extends from the rocky headland at Point Lobos

to the sandy bluffs of Fort Funston (Figures 1 and 3). The back

beach position is variably buttressed by seawalls (2.5 km,

northern end and central), dunes (2.2 km, north–central and

south–central), revetments or low bluffs (1.0 km, southern end,

hot-spot region), and high bluffs (1.3 km, extreme south end).

Based on a 150-year study of shoreline change, the southern

section of Ocean Beach has a long-term erosion trend (Dallas

and Barnard, 2011). Hansen and Barnard (2010) documented

accelerated erosion in recent years, culminating in extensive

shoreline and bluff retreat observed during the powerful El

Nino winter storm season of 2009–10 (Barnard et al., 2011;

Barnard, Hoover, and Hansen, 2011) that caused the partial

collapse of a major roadway and destruction of a parking lot

(Figure 3, upper right). The shoreline of Ocean Beach was

extended seaward to accommodate expansion of the City of San

Francisco in the early 20th century. At the southern end of

Ocean Beach, a nontidal outlet from Lake Merced was filled in

and the shoreline was built out to allow construction of the

Great Highway in the 1920s (Figure 3, bottom).

METHODS

What follows is a brief summary of the methods employed in

this study. For more detailed information, refer to the following

references: Barnard et al. (2007a, 2007b, 2009); Barnard,

Erikson, and Hansen (2009); Barnard and Warrick (2010); Dallas

and Barnard (2009, 2011); and Hansen and Barnard (2010). A

summary of the data collection locations is displayed in Figure 4.

Beach Mapping and Shoreline Change

Subaerial beach mapping was performed at least monthly at

Ocean Beach between 2004 and 2010 using an all-terrain

vehicle (ATV) with a real-time kinematic global positioning

system (RTK-GPS), with additional pre- and poststorm surveys

conducted in the winters of 2005–06, 2006–07, and 2009–10.

The vertical uncertainty of the ATV-collected raw data points is

typically less than 5 cm, and the maximum mean systematic

offset of intersurvey ground control points (raw and gridded) is

less than 3 cm (Barnard et al., 2009). Grids are generated from

the point data, and both shoreline and volume changes between

surveys are calculated. Hansen and Barnard (2010) found a

significant correlation between the mean high water (MHW)

shoreline position and volume at Ocean Beach. Therefore, we

report only MHW shoreline change in this study. Given typical

slopes in the region (tanb 5 0.04), the mean positional

uncertainty of a shoreline is about 0.75 m.

Nearshore Mapping

The coastal profiling system (CPS), a hydrographic survey-

ing system mounted on a personal watercraft (MacMahan,

2001; Ruggiero et al., 2005), was used to perform bathymetric

Figure 2. Wave data from the study area collected during the study period

from April 1, 2004, to March 31, 2010. Top: Polar plot of wave height and

direction for the three wave buoys in the region (see Figure 1 for locations).

Bottom: Wave parameters during the study period. The San Francisco buoy (SF

Buoy-Shelf) only began reporting directional wave data in January 2007, and

the San Francisco bar buoy (SF Bar Buoy) was first deployed in July 2007.

Table 1. Summary of wave statistics during the study period from April 1,

2004, to March 31, 2010.

Wave Buoy

Depth

(m)

Hs (m) Tp (s) Dp (u)

Mean Max Min Mean Max Mean

Deep water 550 2.5 9.1 0.6 11.7 25.0 292

Shelf 55 1.9 8.6 0.4 11.3 23.5 279

Bar 15 1.8 6.8 0.4 12.1 25.0 257

Study of an Erosion Hot Spot 905


surveys along Ocean Beach between 2004 and 2010 approxi-

mately quarterly. The CPS collects accurate position and depth

data at 10 Hz using RTK-GPS and a single-beam echo sounder

to produce bathymetric profiles in areas inaccessible to larger

boats. The vertical uncertainty of CPS-collected soundings is

typically less than 10 cm, although intrasurvey cross-track

comparisons show less than 1-cm mean variability. CPS

surveys at Ocean Beach are performed along 18 1.5-km cross-

shore and 2 alongshore transects (Figure 4) in water depths of

about 1 to 12 m to quantify profile evolution, bar movement,

and cross-shore sediment transport.

The mouth of San Francisco Bay (154 km2) was mapped in

2004–05 using a Reson 8101 multibeam sonar system aboard

the R/V VenTresca operated by the Sea Floor Mapping Lab at

California State University, Monterey Bay. More than 1.2

billion soundings were collected during 44 days of surveying.

Horizontal and vertical positional accuracy of this system is

typically 61 to 2 and 60.12 m, respectively. Due to the density

of data, gridding error is insignificant; thus, the overall vertical

uncertainty is about 12 cm.

Grain Size

Offshore sediment samples were collected annually from

1997 to 2008 at 56 stations at the mouth of San Francisco Bay

by the San Francisco Public Utilities Commission (Figure 4).

Each sample is a composite of two surface grabs taken at each

site using a Smith-McIntyre grab and a 5-cm minimum depth of

penetration criterion. The top 2 cm (1997–99) or top 5 cm

(2000–09) from the two grabs were combined and homogenized

in the field. Grain size was analyzed via dry sieving on a shaker

table. Silt and clay were individually separated by hydrometer

analysis from 1997 to 2004; however, since 2005, they have not

Figure 3. Erosion at the south end of Ocean Beach. Upper left: View south from the northern end of Ocean Beach. Upper right: Bluff erosion in January 2010

within the erosion hot spot. Bottom: Historical shoreline positions at the southern end of Ocean Beach. The area of critical erosion shown is with the white box

(see Figure 1 to reference the location). Lake Merced is pictured to the SE of the hot spot.



been separated, so only percent mud was reported. In summer

2005, 91 grab samples were collected from the region

(Figure 4), with grain size determined using a settling tube

(Gibbs, 1972). In 2008 and 2010, an additional 227 samples

were collected, with the results determined by coulter laser.

Oceanographic Instrumentation

Acoustic Doppler current profilers (ADCPs) were deployed

during six separate experiments (Table 2 and Figure 4) to gain a

better understanding of wave and current variability locally and

regionally, as well as provide calibration and validation data sets

for numerical models. Additional specific goals of these efforts

were to quantify wave transformation across the San Francisco

bar (SF Bar Summer and SF Bar Winter in Figure 4), alongshore

wave variability nearshore at Ocean Beach (SF Bar Summer, SF

Bar Winter, and the Ocean Beach experiment, or OBEX, in

Figure 4), and the discharge and total suspended mass flux of

sediment across the inlet throat (Golden Gate in Figure 4).

Except for the Golden Gate experiment, which utilized a boat-

mounted ADCP and cross-channel transects to measure dis-

charge and sediment flux across the inlet throat, all other ADCPs

were bottom mounted, were upward looking, and measured both

waves and currents. The most extensive instrument deployment

was the OBEX (Figure 4), which consisted of three ADCP

Figure 4. Summary of data collection at Ocean Beach. The main map and the inset include 10- and 1-m bathymetric contours, respectively. For SF Bar Winter,

instruments at the northern and southern ends of the beach were colocated with the SF Bar Summer sites and therefore are not shown on the maps.

Table 2. Physical process measurements at the mouth of San Francisco Bay and offshore Ocean Beach from 2005 to 2010.

Experiment Date Instruments Goals/Measurements

SF Bar Summer Summer 2005 ADCP (4) Regional summer wave and current variations, model testing

SF Bar Winter Winter 2006 ADCP (3) Regional winter wave and current variations, model testing

OB Surf Zone February 2006 ADCP (6) Ocean Beach surf zone circulation patterns, model testing

Golden Gate Winter 2008 ADCP (boat mounted) Golden Gate discharge/sediment flux, model testing

OB Rivera Winter 2008 ADCP (1) Wave conditions on bar peak, model testing

OBEX Winter 2010 ADCP/ADV (9), pressure sensor (6) Alongshore wave height and setup variation



instruments deployed along the approximately 11-m depth

contour over a 1.2-km alongshore stretch, and six inshore surf-

zone stations with buried pressure sensors and acoustic Doppler

velocimeters (ADVs) or ADCPs mounted on poles within the

water column. The goal of OBEX was to make observations of the

hypothesized alongshore variability in wave height and wave

setup along the attachment point of the southern lobe of the ebb-

tidal delta. See Barnard et al. (2007a) and Jones (2011) for more

details on these experiments.

Numerical Modeling

Three numerical modeling approaches were applied to

investigate the potential causes of erosion at Ocean Beach:

(1) Simulating wave nearshore (SWAN) wave modeling (Booij,

Ris, and Holthuijsen, 1999; Holthuijsen, Booij, and Ris, 1993;

Ris, Holthuijsen, and Booij, 1999) was utilized to investigate

the alongshore variation in wave energy at Ocean Beach

(Eshleman et al., 2007), as well as the change in nearshore

wave energy due to multidecadal changes in ebb-tidal delta

bathymetry (Dallas and Barnard, 2009, 2011).

(2) Delft3D hydrodynamic and sediment transport modeling

(Lesser et al., 2004) was coupled with the nearshore wave

model SWAN to investigate the alongshore momentum

balance and sediment transport gradients along Ocean

Beach (Hansen et al., unpublished data), with particular

focus on the influences of the bathymetry of the San

Francisco bar and sewage outfall pipe on the processes

causing the erosion hot spot (Hansen et al., 2011).

(3) The nearshore community model (NearCoM; Shi et al.,

2005) was coupled with SWAN and the quasi–three-

dimensional nearshore circulation model SHORECIRC (Shi

et al., 2003; Svendsen, Haas, and Zhao, 2000) to investigate

wave focusing along Ocean Beach (Shi et al., 2011).

Historical Shoreline and Bathymetric Changes

Long-term (1850s or 1890s–2002) and short-term (1960s or

1980s–2002) shoreline change was evaluated from just east of

the Golden Gate Bridge to Point San Pedro (,30 km; Figure 1).

Existing digital shorelines were acquired from Hapke et al.

(2006) and the National Oceanic and Atmospheric Adminis-

tration Shoreline Data Explorer (NOAA, 2009). The data are

originally from topographic sheets, digital raster graphics, and

light detection and ranging (LIDAR) data sets. In addition,

aerial imagery (1983) and LIDAR data (1997, 1998, and 2002)

were used to supplement the existing shorelines. Short-term

endpoint shoreline change rates were calculated at 50-m-

spaced transects comparing the 1960s or 1980s and 2002

shoreline positions. Long-term rates of shoreline change were

Figure 5. Alongshore variations in recent shoreline change (left), and foreshore slope and beach width (right). The erosion hot spot is centered at Northing

4176 km and is approximately 1 km in length.



calculated using linear regression applied to all shorelines from

the earliest rates (1850s or 1890s) to 2002.

Sounding data from four historic bathymetric surveys (1873,

1900, 1956, and 2005) were used to create bathymetric grids of

the San Francisco bar (Dallas and Barnard, 2009, 2011). For

the 1873 and 1900 grids, soundings were digitized from

hydrographic sheets obtained from the National Ocean Service

(NOS). Sounding data were then registered to a common

horizontal datum using the intersection of latitude and

longitudinal lines (i.e., graticules). For the 1956 and 2005

surveys, registered soundings were obtained directly from the

NOS and from the California State University, Monterey Bay

Sea Floor Mapping Lab, respectively. Bathymetric grids with a

horizontal resolution of 25 m were generated for each survey.

Grids were adjusted to a common vertical datum (NAVD88) to

account for changes in sea-level rise (i.e., tidal epoch and tidal

datum) and differenced to create bathymetric change grids.

SYNTHESIS OF LOCAL ASSESSMENTS

Beach Morphology and Recent Shoreline Change

The topographic surveys at Ocean Beach between 2004 and

2010 document substantial spatial and temporal variability in

the subaerial beach morphology (Figure 5). In addition to

seasonal changes that can amount to nearly 100 m of MHW

shoreline change associated with seasonal variability in wave

height, the shoreline at Ocean Beach exhibited a strong pattern

of counterclockwise rotation during the study period, with the

north end of the beach accreting while the south eroded

(Figure 3, bottom; Hansen and Barnard, 2010). Empirical

orthogonal function (EOF) analysis indicates that seasonal

shoreline change is the dominant signal in the shoreline

position data set, accounting for about 56% of the total

Figure 6. Variations in cross-shore morphology from CPS data lines 3

(northern end), 7 (central, on longitudinal bar), 13 (southern, hot spot), and 17

(extreme southern end) from 2004 to 2011 (see Figure 4 for transect locations).

Figure 7. Nearshore bathymetric change from November 2004 to October

2010. The coordinate system is eastings (x) and northings (y) in the

Universal Transverse Mercator (UTM) projection.



variance. The second EOF mode, accounting for about 16% of

the variance, is the rotational signal in the shoreline. For

Ocean Beach, the rotational signal accounts for an increasing

percentage of the variance as the temporal span of the

shoreline record length increases.

Beach width and foreshore slope along Ocean Beach also

vary considerably (Figure 5, right). At the north end of the

beach, the mean beach width exceeds 160 m in some areas,

while in the more erosional area in the south, the mean beach

width is only about 30 m. The MHW shoreline eroded as much

as 32 m in the erosion hot-spot area between 2004 and 2010

(Figures 3, bottom, and 5). Additional landward erosion in the

hot-spot area is limited due to armoring (e.g., Figures 5,

Northing 4176, and 3, bottom). The impact of the armoring is

particularly evident in the foreshore slope at Northing 4176 km

(Figure 5).

Figure 8. Top: Locations of summer 2005 and winter 2010 (OBEX) deployments. The 10- and 15-m contours are shown in black (2-m intervals to 20 m on the left

panel, 1-m intervals on the right panel). Bottom: Mean, max, and minimum surface (S), bottom (B), and depth-averaged (D) alongshore currents measured

during summer 2005 (lower left). The polar plot of wave direction and height for the summer 2005 deployment at sites 4 and 3 is given at the lower right.



Nearshore Morphology and RecentBathymetric Change

The shape of the nearshore profile and the intra-annual

variability is controlled to varying degrees by both wave

exposure and location within the ebb-tidal delta complex along

Ocean Beach (Figure 6). The flattest profiles with the greatest

intra-annual variability occur in the center (e.g., Figure 6, line

7), where the longitudinal bar attaches to the beach and the

largest waves are found (Figure 1). Conversely, the steepest

and least variable profiles are located at the extreme southern

end of the beach (e.g., Figure 6, line 17), in the shadow of the

ebb-tidal delta. The profiles in the erosion hot-spot region (e.g.,

Figure 6, line 13) are intermediate between the central portion

and the extreme southern end of the beach, with moderate

slopes and intra-annual variability. Profiles at the northern

end of the beach (e.g., Figure 6, line 3) are subjected to strong

tidal influence, with a large tidal channel cutting directly

through the nearshore region, but also exhibit evidence of

strong alongshore transport during extreme events (Barnard,

Hoover, and Hansen, 2011). Bathymetric change from the CPS

surveys between November 12, 2004, and October 28, 2010, is

shown in Figure 7. The bathymetric survey region experienced

mean vertical accretion of 0.22 m, which equates to a total

volume change of +1.95 million m3. A significant amount of this

accretion can be attributed to the extreme net nearshore

accretion observed during the recent El Nino year (May 2009–

May 2010), a total of 1.6 million m3 of sediment (Barnard,

Hoover, and Hansen, 2011). However, the spatial patterns of

bathymetric change are consistent with the observed shoreline

trends from 2004 to 2010 (Figure 5) and from 2004 to 2009

(Hansen and Barnard, 2010). For example, at the northern end

of the beach, high rates of shoreline accretion correlate with

high rates of nearshore accretion, indicative of the overall

counterclockwise rotation of both the beach and the nearshore

regions. Erosion generally dominates the extreme nearshore

(i.e., depths of ,5 m) in the central portion of the beach, while a

narrow but prominent pocket of erosion is located immediately

adjacent to the erosion hot spot. Significant amounts of

accretion are present offshore and to the south of the

longitudinal bar and the erosion hot spot, the latter being

influenced by annual nearshore dredge disposal placement

from 2005 to 2010 (Barnard, Erikson, and Hansen, 2009;

Barnard et al., 2009). Overall, the observed bathymetric

change from 2004 to 2010 is indicative of sediment loss from

the beach and gain to the extreme northern part of the beach

and offshore.

Alongshore Variability in Physical Forcing

Currents and Waves

In summer 2005, three ADCPs were deployed in the

nearshore at Ocean Beach, and an additional instrument was

placed on the NW edge of the ebb-tidal delta (Figures 4 and 8).

Sites 1 (north end) and 2 (north–central) were north of the

longitudinal bar, and site 3 (south end) was just offshore of the

erosion hot-spot region. The northern section of the beach

experienced strong alongshore currents, peaking at nearly 2 m/

s at the surface during the flooding stage (mean depth average

0.4 m/s), whereas the north–central and southern stretches had

mean current magnitudes that were less than half as strong

(Figure 8). However, this relationship holds only on the

flooding tide. On the ebbing tide, current speeds at the south

end (site 3) can equal or exceed those measured in the northern

section of the beach, likely due to the ebb being dominated by a

jet that extends straight west of the inlet, thereby limiting the

currents along Ocean Beach (Barnard et al., 2007a). These

currents flowed along the coast, were dominated by tidal

forcing, and had a very small landward component of flow. The

strongest currents were at the surface, and vertical gradients

in current magnitude showed velocities varying by as much as

1 m/s throughout the water column, with an average vertical

range of velocities on the order of 0.25 m/s (Figure 8).

While the northern portion of the beach experienced stronger

currents overall, wave energy was much greater in the

southern portion near the erosion hot spot. This is reflected

in the higher measurements of significant wave height at site 3

compared to sites 1 and 2 (Barnard et al., 2007a). Site 4 had

higher angles of incidence by approximately 30u than directions

Figure 9. SWAN predicted alongshore variations in wave height at the

10-m contour at Ocean Beach from three wave cases. The model was forced

with 2.5-m offshore significant wave height and 14-s peak period but with

differing directions of 272u, 292u, and 311u.



measured at the inshore sites due to refraction across the ebb-

tidal delta (Figure 8).

The spatial variability in wave conditions along Ocean Beach

due to refraction across the ebb-tidal delta was investigated

using the SWAN wave model. Tide-cycle (24.8 h), averaged,

predicted significant wave heights from a coupled SWAN–

Delft3D model of three common offshore wave cases, all with an

offshore significant wave height of 2.5 m and a period of 14 s but

with the differing directions of 272u, 292u, and 311u, are shown

in Figure 9. Waves that approach the coast from the W (,270u)and W–NW (,290u) are strongly focused by the ebb-tidal delta,

resulting in considerable alongshore variability in wave height

along Ocean Beach. The largest waves occur around Northing

4178 to 4179 km, where the longitudinal bar merges with the

shoreline in the middle of Ocean Beach. Wave focusing by the

ebb-tidal delta can result in a more than 70% increase in wave

energy from the south end of Ocean Beach to the wave focal

zone. Waves that approach the coast from the NW (,315u) are

less focused and more dissipated by the ebb-tidal delta prior to

reaching Ocean Beach, and they do not exhibit as much

alongshore variability in wave energy (Figure 9).

Field measurements obtained during the OBEX experiment

(Figure 8) confirm these findings. During the 4-month-long

deployment, offshore significant wave heights up to 6 m were

recorded. The most northerly instrument at site 7, closest to the

apex of the longitudinal bar, consistently recorded wave heights

larger than the sites to the south, with as much as a 40% greater

wave height between sites 7 and 9 (mean difference 0.18 m;

Figure 10). Correspondingly, the inshore, surf-zone pressure

sensors recorded a similar pattern of wave-induced setup during

the 2-week deployment, with water levels about 10 cm higher in

the north, compared to 600 m farther to the south (Hansen et al.,

unpublished data; Jones, 2011). The alongshore variation in

wave height and the corresponding wave-induced setup result in

significant pressure gradients that sometimes dominate near-

shore circulation patterns. Even during moderate conditions at

Ocean Beach (deep-water significant wave height [Hs] 5 3.5 m,

peak wave period [Tp]5 15 s, and peak wave direction [Dp] 5

270u), NearCoM results indicate that setup can vary by 30 cm

alongshore—0.4 m in the center of the beach near the wave focal

point, 0.1 m in the extreme south (Shi et al., 2011). Alongshore

variability of forcing in the surf zone has direct implications on

the persistence of the hot spot, leading to gradients in the velocity

field and thus transport of sediment.

Nearshore Hydrodynamics and Effects of theExposed Outfall Pipe (Delft3D)

A 200-m-wide trough in about 12- to 16-m water depth is

associated with the scour of an exposed rock crown covering an

outfall pipe offshore of the erosion hot spot (Figures 1 and 11).

Figure 10. Comparison of significant wave heights measured at sites 7 and 9 (see Figure 8, top, for instrument locations). The red curve is the significant

wave height measured at site 9; the black curve is the significant wave height at site 7. The black curve in bottom panel shows the percent difference between

wave heights at the two sites.



The scour was observed during a multibeam bathymetry survey

of the entire mouth of San Francisco Bay in 2004–05 and

subsequently confirmed by more than a dozen surveys exclusive-

ly in the outfall pipe vicinity from 2005 to 2010 (Barnard,

Erikson, and Hansen, 2009; Barnard et al., 2007a, 2007b, 2009).

Delft3D was used to analyze the alongshore momentum

balances for the entire extent of Ocean Beach (Hansen et al.,

unpublished data) with particular emphasis on exploring the

influence of the ebb-tidal delta and the outfall pipe on the basic

forcing terms (i.e., radiation stress and pressure gradients;

Hansen et al., 2011). The model predicted flow patterns that

were favorable for sediment removal from the erosion hot-spot

area and net erosion from the surf zone. Analysis of the forcing

terms driving surf-zone flows revealed that wave refraction over

the exposed wastewater outfall pipe between the 12- and the 16-

m isobaths introduces a perturbation in the wave field that

results in erosion-causing flows (Figure 12). The model predicts

that the scour adjacent to the outfall leads to upward of a 0.5-m

decrease in local wave heights over the pipe, refracting waves

away from the trough much like a submarine canyon, albeit on a

much smaller scale. Although the scour does not extend inshore

of an approximately 12-m depth, modifications to the wave field

still exist in the surf zone and lead to a large discontinuity in

both pressure and radiation stress gradients. These perturba-

tions in the flow-forcing terms together cause a persistent rip

current and alongshore flow acceleration (Figure 12). These

flows appear under a variety of wave and tidal conditions but

are most apparent when offshore waves approach the coast from

the prevalent W to W–NW directions (,270u–300u). Modeled

potential alongshore sediment transport gradients indicate

sediment removal from the erosion hot-spot area (Figure 12).

SYNTHESIS OF REGIONAL ASSESSMENTS

After focusing on the morphodynamics directly impacting the

erosion hot spot at Ocean Beach due to the outfall pipe and the

shape of the ebb-tidal delta, it is important to describe this small

Figure 11. Detail of the bathymetry at the southern end of Ocean Beach (top) with the cross-section across the outfall pipe from the May 2005 multibeam

survey (bottom). The coordinate system is eastings (x) and northings (y) in the UTM projection.



area in the context of the evolution of the entire coastal system.

The Golden Gate inlet is the sole connection linking the open

coast (i.e., the ebb-tidal delta and adjacent beaches) with the San

Francisco Bay estuary, collectively referred to as the San

Francisco Bay coastal system. The Golden Gate provides the

conduit for the daily transport of about 8 trillion L of water (8 3

109 m3, 93,000 m3/s), which carries mud, sand, biogenic material,

nutrients, and pollutants in and out on the tides. Therefore, any

significant changes to San Francisco Bay and the 29 watersheds

that drain into it, particularly modifications to the tidal prism,

sediment supply, or freshwater discharge, can significantly

influence coastal processes at the mouth of San Francisco Bay,

including the ebb-tidal delta and adjacent open-coast beaches.

Historical Changes to the San Francisco BayCoastal System

The San Francisco Bay coastal system has been significantly

impacted by anthropogenic activities since the mid-19th century

(Figure 13). Hydraulic mining during the Gold Rush released

about 850 million m3 of sediment into San Francisco Bay

watersheds in the second half of the 19th century (Gilbert,

1917). Bay development, including the filling or diking of 95% of

the tidal marsh areas from 1850 to the late 20th century,

reduced the tidal exchange surface area by about two-thirds

(Atwater et al. 1979) and tidal prism by about 10% (Conomos,

1979; Dallas and Barnard, 2011; Gilbert, 1917; Keller, 2009).

Over the last century, a minimum of 200 million m3 of sediment

has been permanently removed from the San Francisco Bay

coastal system through dredging, aggregate mining, and borrow

pit mining, including at least 54 million m3 of sand-sized or

coarser sediment from central San Francisco Bay, immediately

adjacent to the Golden Gate inlet (Dallas and Barnard, 2011). In

addition, Wright and Schoellhamer (2004) demonstrated that

modifications to the Sacramento and San Joaquin River deltas,

the primary watersheds feeding San Francisco Bay, have

resulted in an approximately 50% reduction in suspended

sediment flux to the bay from 1957 to 2001. The waning of the

hydraulic mining signal by the mid-20th century (Porterfield,

1980), coupled with delta modifications and direct removal of

sediment from the bay floor, is reflected in the volumetric

seabed change that has been recorded from San Francisco Bay

(Capiella et al., 1999; Fregoso, Foxgrover, and Jaffe, 2008; Jaffe,

Smith, and Torresan, 1998) and the ebb-tidal delta (Dallas and

Barnard, 2011; Hanes and Barnard, 2007) over the last 50 years:

a total of about 240 million m3 of sediment loss, including 14

million m3 of predominantly coarse sediment loss in just the last

decade from west–central San Francisco Bay, linked primarily

to aggregate mining (Barnard and Kvitek, 2010).

Very large, ebb-dominated bedforms were mapped during the

2004–05 multibeam survey in the inlet throat (Barnard et al.,

2006), and an extensive study of bedform asymmetry (a proxy

for bedload transport) throughout the mouth of San Francisco

Bay and the west–central San Francisco Bay (Barnard et al.,

2012) indicates that net potential sand transport along the

seabed is ebb directed (Figure 14, top). In addition, boat-

mounted, cross-channel ADCP transect-calculated discharge

and sediment flux across the inlet throat clearly supports the

hypothesis that tidal sand transport potential is ebb dominated.

This finding was confirmed by a numerical modeling exercise in

which the annual sand transport volumes were estimated

based on a 36-day hydrodynamic tidal simulation (no wave

modeling) that is a proxy for annual behavior (Barnard et al.,

2012, Figure 14, bottom). Therefore, if sediment availability in

San Francisco Bay becomes progressively limited, less sedi-

ment will be transported seaward to the ebb-tidal delta and

open-coast beaches. Given the many factors that have limited

sediment availability in San Francisco Bay over the last

century (e.g., decay of the hydraulic mining signal, damming

and diversion of sediment in the San Joaquin River and

Sacramento River deltas, widespread development of wetlands,

borrow pit mining, dredging, and aggregate mining), in

addition to the continued removal of coarse sediment by

aggregate mining at a rate of about 1 million m3/y, there is

every indication that this trend of sediment loss will continue.

Furthermore, with projections of sea level rise (e.g., Vermeer

and Rahmstorf, 2009) likely to increase the depth of San

Francisco Bay over the coming decades, the estuary would

become a more efficient sediment sink, though localized scour

potential may increase due to the enlarged tidal prism.

Nevertheless, these factors generally indicate that the potential

rate of coarse sediment transport from the bay to the ocean will

certainly not accelerate over recent rates and will probably

continue to decline for the foreseeable future.

Figure 12. Map view of the flow patterns at four tidal stages offshore of the

erosion hot spot and onshore of the wastewater outfall pipe, showing the

persistent rip current that develops as a result of the perturbation in the wave

field caused by scour surrounding the pipe. The vertical bars in the lower panel

indicate the tidal stage of the four output times shown (Hansen et al., 2011).



Ebb-Tidal Delta Evolution

Similar to most regions inside San Francisco Bay, from 1956

to 2005, the mouth of San Francisco Bay lost sediment (292

million m3; Hanes and Barnard, 2007), in particular the ebb-

tidal delta (276 million m3; Dallas and Barnard, 2011;

Figure 15). The pattern of change can be characterized by

ebb-tidal delta contraction whereby the outer lobe eroded

heavily, with some accretion on the interior. A radial pattern of

contraction has been prevalent during the other survey

intervals as well (i.e., 1873–1900 and 1900–56), even during

an episode of volumetric accretion. Because there has been no

significant change in the wave climate to cause contraction, the

period of accretion (1900–56) is linked to an increase in

sediment supply, coupled with a reduction in tidal prism

(Dallas and Barnard, 2011). During the last half of the 20th

century, minimal infilling of the bay occurred, so the pervasive

erosion and contraction of the ebb-tidal delta from 1956 to 2005

is attributed primarily to a reduction in sediment supply to the

mouth of San Francisco Bay.

The pattern of ebb-tidal delta contraction has direct implica-

tions on the southern end of Ocean Beach in the vicinity of the

erosion hot spot. The southern outer lobe of the ebb-tidal delta

that formerly provided protection to southern Ocean Beach from

direct wave attack in 1956 migrated about 1 km to the north by

2005, leaving the seabed about 1 to 2 m deeper offshore of the

present erosion hot spot (Figure 15, inset). Using SWAN, Dallas

and Barnard (2011) showed that the change in bar bathymetry

from 1956 to 2005 resulted in an approximately 10% increase in

wave power in the erosion hot-spot region during typical winter

storms. The greater depth reduces nearshore wave energy

dissipation; therefore, larger waves can directly impact this

section of coastline. In addition, wave-induced onshore sediment

transport that has been hypothesized to occur along the crest of

the ebb-tidal delta toward Ocean Beach (Battalio and Trivedi,

1996) no longer has the potential to directly feed this region.

Figure 13. Overview of historical changes to the San Francisco Bay coastal system. See the text for references to reported bathymetric change and

anthropogenic influences.



Figure 14. Evidence for potential ebb-directed sediment transport. Top: Bedform asymmetry indicating ebb-dominated transport through the Golden Gate.

Bottom: Delft3D-modeled residual tidal transport, also indicating ebb dominance (both plates modified from Barnard et al., 2012).



Regional Shoreline Changes

The radial contraction and erosion of the ebb-tidal delta and

systemwide sediment loss are also reflected in the short- and

long-term trends of shoreline change along the outer coast

adjacent to the mouth of San Francisco Bay (Figure 16).

Shoreline change rates were calculated for long-term (1850s–

90s to 2002) and short-term (1960s–80s to 2002) periods.

In the shadow of the ebb-tidal delta along the San Francisco

shoreline, from Crissy Field to northern Ocean Beach, the

majority of the coastline was accreting in the long term (86%) and

short term (87%), with the rate of accretion accelerating threefold

to +0.6 6 0.4 m/y. Accretion also dominated the northern half of

Ocean Beach during both periods, suggesting more sediment is

available for northerly transport, possibly due to flood channel

infilling and ebb-tidal delta contraction, both of which could

result in an increase in sediment supply to this area.

However, from the middle of Ocean Beach south to the end of

the littoral cell at Point San Pedro, erosion dominates the

shoreline change signal. Nearly the entire coast south of the

ebb-tidal delta (San Mateo) was heavily eroding in the long

term (93% of all transects) and short term (98% of all

transects), with the rate increasing by 50% (20.6 6 0.3 m/y)

in the last several decades. In addition, shoreline change

results for the state of California by Hapke et al. (2006) showed

that the stretch of coastline from Point Lobos (i.e., the northern

boundary of Ocean Beach) to Davenport (,80 km south of

Point San Pedro), which includes the Ocean Beach and San

Mateo regions covered in this study, has the highest regionally

averaged long-term erosion rate in the state. Together, these

results indicate widespread erosion along the outer coast

adjacent to the mouth of San Francisco Bay, with an increase

in erosion rates in recent decades. The area south of Ocean

Beach has always been beyond the direct influence of

significant tidal current sediment transport from San Fran-

cisco Bay, so the likely cause of the erosional trend is a

reduction in the sediment supply to the region. Eustatic sea

level rise has been suppressed along the U.S. West Coast since

Figure 15. Bathymetric change at the mouth of San Francisco Bay from 1956 to 2005. In the inset, the coordinate system is eastings (x) and northings (y) in

the UTM projection.



1980 (Bromirski et al., 2011) and therefore is ruled out as a

factor in any recent shoreline changes. The transition from

trends of accretion to trends of erosion at Ocean Beach occurs

where the crest of the ebb-tidal delta attaches to the shoreline

(Figure 17). Therefore, it appears that the shape of the ebb-

tidal delta, in particular the location of the crest, exerts

dominant control on shoreline change at Ocean Beach. Dallas

and Barnard (2011) showed that in the central and southern

sections of Ocean Beach, shoreline change and significant

wave height change at the 10-m contour are well correlated for

winter storm conditions from the 1950s to the 2000s, with an

increase in nearshore wave height linked to shoreline erosion

and the highest correlation in the vicinity of the erosion hot

spot. Given that some climate models suggest an increase in El

Nino–like conditions in California over the coming decades,

possibly resulting in more frequent and intense storms (e.g.,

Cayan et al., 2008), as well as recent trends of increasing wave

heights (especially extreme waves) for much of the California

coast (Allan and Komar, 2006; Wingfield and Storlazzi, 2007),

there is every indication that increasingly powerful waves will

impact the exposed southern portion of Ocean Beach.

Decadal Grain Size Variations

At the mouth of San Francisco Bay, mean grain size generally

correlates with tidal velocity magnitude, fining from coarse sand

and gravel in the inlet throat (i.e., the Golden Gate) to very fine

sand on the outer reaches of the ebb-tidal delta and inner

continental shelf (Figure 18). As the delta contracts, as a result of

decreasing sediment supply, weaker tidal currents due to tidal

prism reduction, or both, the outer reaches should become

progressively finer. This could be a result of either finer shelf

sediment being exposed or transported shoreward by wave action

or finer sediment coming out of the bay. Regardless, annual

sediment sampling from up to 56 stations between 1997 and 2008

shows a fining of mean grain size by about 25 mm (Figure 18b).

This fining is also reflected in the percent mud (Figure 18c) and

percent sand (Figure 18d) content of the samples. Between 2002

and 2007, there was a substantial shift to progressively finer

sediment along the outer reaches of the ebb-tidal delta. Of

particular note is the pervasive fining to the north and south of

the ebb-tidal delta and the much finer sediment immediately

offshore of the erosion hot spot. Finer, more easily erodible

sediment offshore of southern Ocean Beach could be an

additional contributing factor to the vulnerability of the site.

The regional-scale fining of sediment at the mouth of San

Francisco Bay from 1997 to 2008 may be related to the observed

36% step decrease in suspended sediment concentrations (SSCs)

observed inside the bay between the 1991–98 and the 1999–2007

water years, broadly attributed to the depletion of the ‘‘erodible

sediment pool’’ that was once filled with pulses of sediment due to

the effects of hydraulic mining and urbanization, and further

reduced by sediment trapping behind dams (Schoellhamer,

Figure 16. Regional historical shoreline changes. Distance is from the San

Francisco Marina inside the estuary (modified from Dallas and Barnard, 2011).

Figure 17. Historical shorelines changes along Ocean Beach. The long-

term rate (linear regression) is from 1899 to 2002, and the short-term rate

(end point) is from 1983 to 2002. The coordinate system is eastings (x) and

northings (y) in the UTM projection.



2011). The decrease in SSCs, and presumably by proxy in coarser

bedload, could affect the mouth of San Francisco Bay in two

primary ways. First, this flushing of erodible, mostly fine

sediment could have been transported seaward through the

Golden Gate inlet, settling on the distal edges of the ebb-tidal

delta and vicinity and reducing the bed grain size. Second, the

observed reduction of suspended sediment from 1991 to 2007

inside San Francisco Bay is indicative of a limited supply to the

mouth of San Francisco Bay; therefore, fine, wave-driven shelf

sediment became more prevalent or continued contraction of the

Figure 18. Grain size at the mouth of San Francisco Bay. (a) Gridded grain size from mean of samples collected from 1997 to 2008 (n 5 56), and single

samples collected in 2005, 2008, and 2010 (n 5 385; sample sites shown in Figure 4). (b) Change in mean grain size from 1997 to 2008. (c) Change in percent

mud from 1997 to 2008. (d) Change in percent sand from 1997 to 2008.



ebb-tidal delta exposed finer sediment. Either way, progressively

finer sediment, coupled with an increase in wave energy (see the

previous section), will continue to favor erosion in the southern

portion of Ocean Beach.

FINAL SYNTHESIS

The rotational signal in the MHW shoreline at Ocean Beach

(Figures 5 and 19) directly follows the pattern seen in the ebb-

tidal delta evolution (Figure 15). Transition from shoreline

accretion to erosion along the beach at all temporal scales occurs

at the same approximate alongshore area as the bathymetric

change between 1956 and 2005 (Figure 15), which also switches

from accretion to erosion (Figure 19). The patterns in the MHW

shoreline change, beach width, and change in the ebb-tidal delta

indicate that the delta exerts first-order control on multiannual

beach change. The changes in the subaerial beach follow the

historic changes observed in the ebb-tidal delta. Thus, as the

delta has contracted (Figure 15) the beach in the erosion hot-

spot area has narrowed following the erosion of sediment

offshore of the hot-spot area.

A variety of data collection techniques and numerical

modeling approaches have been integrated to explore the cause

of an erosion hot spot at Ocean Beach. The collective approach

describes a coastal system that is highly variable alongshore,

both in morphology and in physical forcing. The shape of the

ebb-tidal delta and proximity to the tidal inlet are the dominant

controls on the observed variability in nearshore circulation

patterns and coastal evolution. The shape of the ebb-tidal delta,

in turn, has been significantly altered by a massive loss of

sediment, on the order of 0.25 billion m3, from the San Francisco

Bay coastal system over the last half century. Ultimately, the

shape of the ebb-tidal delta, especially the precise location of the

longitudinal bar that dictates the location of intense wave

focusing, determines the nearshore circulation patterns and

areas prone to erosion. The entire stretch of shoreline south of

the longitudinal bar experiences erosion, and points north have

been accreting. If the observed trend of ebb-tidal delta

contraction continues, this nodal point is likely to migrate

northward, exposing more beach to critical erosion. The erosion

hot spot along the 1-km section of coast south of the longitudinal

bar attachment point at Ocean Beach exists for the following

reasons: (1) the shoreline was artificially built out during

expansion of the City of San Francisco in the early 1900s; (2)

systemwide loss of sediment leads to contraction of the ebb-tidal

delta, which in turn reduced sediment supply to the area; (3)

southward decreasing wave heights under most conditions

result in alongshore gradients in the flows and sediment

transport; and (4) the scour of the outfall pipe and associated

rock crown results in a local depression in wave heights,

significantly modifying nearshore circulation patterns that

enhance erosion in that area. The study highlights the

advantage of exploring local erosion with a highly focused

geomorphic and physical processes investigation, as well as a

Figure 19. Left: Alongshore variations at Ocean Beach of regional historical long-term (1899–2002) and short-term (1983–2002) shoreline change rates and

recent (2004–10) shoreline change rates. Right: Comparison of nearshore bathymetric change from 1956 to 2005 along the 10-m bathymetric contour (from

the 2005 multibeam survey) and recent shoreline change rates.



broad, systemwide approach, to fully understand the variety of

potential influences on local coastal behavior.

ACKNOWLEDGMENTS

We thank the U.S. Geological Survey (USGS) Mendenhall

Postdoctoral Fellowship Program, the USGS Coastal and

Marine Geology Program, and the U.S. Army Corps of

Engineers, San Francisco District (especially Peter Mull),

for funding this research. Mike Kellogg of the San Francisco

Public Utilities Commission generously provided sediment

data. The National Park Service provided research permits

and other logistical support. Numerous other individuals

contributed to the development and execution of this research

effort. In particular, we thank Dan Hanes for his guidance

and advice throughout the course of this investigation.

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